SEM_WAIT(3) | Library Functions Manual | SEM_WAIT(3)

# NAME

sem_wait, sem_timedwait, sem_trywait — decrement (lock) a semaphore

# LIBRARY

POSIX Real-time Library (librt, -lrt)

# SYNOPSIS

#include <semaphore.h>

int
sem_wait(sem_t *sem);

int
sem_timedwait(sem_t *sem, const struct timespec * restrict abstime);

int
sem_trywait(sem_t *sem);

# DESCRIPTION

The sem_wait() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until the value is non-zero and the value can be decremented.

The sem_timedwait() function performs the same action, but will not wait beyond abstime for the value of sem to be non-zero.

The sem_trywait() function decrements (locks) the semaphore pointed to by sem only if the value is non-zero. Otherwise, the semaphore is not decremented and an error is returned.

# RETURN VALUES

The sem_wait() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

# ERRORS

sem_wait(), sem_timedwait(), and sem_trywait() will fail if:

[EINVAL]
  sem points to an invalid semaphore.

Additionally, sem_trywait() will fail if:

[EAGAIN]
  The semaphore value was zero, and thus could not be decremented.

sem_timedwait() will also fail if:

[ETIMEDOUT]
  The semaphore value was zero and abstime was reached.

# SEE ALSO

sem_post(3)

# STANDARDS

sem_wait() and sem_trywait() conform to ISO/IEC 9945-1:1996 (“POSIX.1”).

February 29, 2012 | NetBSD 7.2